Triapine is a ribonucleotide reductase (RNR) inhibitor that reduces the cellular pool of DNA precursors (dNTPs) by interfering with their de novo synthesis (Cory et al., (1994) Biochem. Pharmacol. 48, 335-44). Depletion of dNTPs resulted in inhibition of DNA synthesis. Triapine was first developed as an anti-cancer agent for its action against the growth of tumor cells both in vitro and in vivo (Liu et al., (1992) J. Med. Chem. 35, 3672-77). It has recently been shown that concentration of Triapine reached 1.0 micromolar in cancer patients receiving a 96-hour infusion of Triapine at a dose of 96 mg/mm2 (Modiano et al., Proc. Am. Assoc. Cancer Res., 42, 834, 2001). The mechanism of action of Triapine is similar to that of another anti-cancer agent, hydroxyurea, which has been approved for treatment of cancers in humans.
Recently, hydroxyurea, another known RNR inhibitor, was shown to have synergistic effects against HIV-infected cells (human immunodeficiency viruses, the causative agents of AIDS) when combined with 2′,3′-dideoxyinosine (DDI) (Gao et al., (1998) Biochem. Pharmacol. 56, 105-12). The mechanism of action is unknown, but may be due to the depletion of dNTPs in cells treated with hydroxyurea.
3-AP and 3-AMP, like other thiosemicarbazone analogs of this class (Springam and Sartorelli, J. Med. Chem. (1979) 22, 1314-6), have very strong iron binding affinity and are capable of removing iron from ferritin. Iron is required RNR activity and for normal physiological function of organisms and iron deprivation inhibits proliferation of protozoa (Merali et al., Antimicrob. Agents Chemother. (1996) 40, 1298-1300), bacteria (Lowy et al., Antimicrob. Agents Chemother. (1984) 25, 375-6), fungi (Newman et al., Antimicrob. Agents Chemother. (1995) 39, 1824-9; Shulman et al., Arzneimittelforschung (1972) 22, 154-8; Kerbs et al., Sabouraudia (1979), 17, 241-50), and viruses (Dai et al., Virology (1994) 205, 210-6; Cinatl et al., Antiviral Res. (1994) 25, 73-77; Bayraktar et al., J. Viral Hepat. (1996) 3, 129-35; Martelius et al, Transplantation (1999) 15, 1753-61; Georgiou et al, J. Infect. Dis. (2000) 181, 484-90). In addition to depletion of intracellular dNTP pools, that 3-AP inhibits viral dissemination could be mediated through its iron chelating properties (Chouteau et al., J. Hepatol. (2001) 34, 108-13; Georgiou et al, J. Infect. Dis. (2000) 181, 484-90; Bayraktar et al., J. Viral Hepat. (1996) 3, 129-35; Conti et al., Boll. Ist. Sieroter Milan (1990) 69, 431-6).
3-AP, because of its strong iron-chelating property, can be used to remove excessive tissue iron in sickle cell disease patients who require regular blood transfusion (Cohen and Martin, Semin. Hematol. (2001) 38(Suppl. 1), 69-72). As a potent inhibitor of RNR, 3-AP could also be used for the treatment of psoriasis (Smith, Clin. Exp. Dermatol. (1999) 24, 2-6).
Chronic HBV (hepatitis B virus) infection remains a therapeutic challenge for clinicians. The recent development of lamivudine has provided new hope in the therapy of chronic hepatitis B. However, due to the slow kinetics of viral clearance and the spontaneous genetic variability of HBV, lamivudine therapy is associated with the selection of drug resistant mutants in up to 50% of patients after 3 years of therapy. It is therefore important to continue research to develop new anti-HBV strategies using in vitro and in vivo evaluation in experimental models of HBV replication.
Herpes simplex virus (HSV) encodes a RNR which is similar to the one encoded by mammalian cells. HSV replication does not require the expression of viral RNR in exponentially growing cells but is required for viral replication in quiescent cells (Goldstein and Weller, (1988) Virol. 166, 1-51). Duan et al., Antimicrob. Agents Chemother. (1998) 42, 1629-35, showed that the RNR inhibitor BILD1633 SE in combination with acyclovir had activity against acyclovir-resistant HSV strains.
Recently, flavivirus infections, including West Nile virus infections, have become increasingly frequent in the United States. The flaviviruses are agents of infectious disease which predominate in East, Southeast and South Asia and Africa, although they may be found in other parts of the world as well. Japanese encephalitis virus is the causative agent of Japanese encephalitis (JE). The mortality rate from JE is rather high and the disease brings heavy sequelae. Although found in Japan, the disease has spread to other parts of Asia and is now found predominantly outside of Japan, primarily in South and Southeast Asia.
Dengue viruses are causative agents of dengue fever/dengue hemorrhagic fever. Infection with dengue viruses is a major public health problem in tropical countries, expecially in Southeast Asia and the Western Pacific, but dengue viruses may also be found in the Americas. As the dengue virus is transmitted to humans via the Aedes aegypti mosquito, it is not unexpected that tropical and subtropical countries, in particular, those in Southeast Asia, are highly endemic for dengue.
Viral replication requires dNTPs, and depletion of intracellular dNTPs by Triapine may prevent viruses from multiplying. In addition, this new strategy may be used in combination with other anti-viral agents to treat, or to prevent or delay the development of drug resistant mutants.